May I ask a question about process programming?

I would like to ask a question about process programming-Linux general technology-Linux programming and kernel information. The following is a detailed description. The GNU/Linux programming guide version 2nd contains a sample subroutine in Process Control. The source code getname. c is as follows:

# Include
# Include
# Include
# Include
# Include

Int main (void)
{
Char * login;
Struct passwd * pentry;

/* Get the login name */
If (login = getlogin () = NULL) {/* oops */
Perror ("getlogin ");
Exit (EXIT_FAILURE );
}

/* Get the password entry for login */
If (pentry = getpwnam (login) = NULL ){
Perror ("getpwnam ");
Exit (EXIT_FAILURE );
}

/* Display the password entry */
Printf ("user name: % s \ n", pentry-> pw_name );
Printf ("UID: % d \ n", pentry-> pw_uid );
Printf ("GID: % d \ n", pentry-> pw_gid );
Printf ("gecos: % s \ n", pentry-> pw_gecos );
Printf ("home dir: % s \ n", pentry-> pw_dir );
Printf ("shell: % s \ n", pentry-> pw_shell );

Exit (EXIT_SUCCESS );
}

This program shows which user is running the process and the output user name, UID, and other information. However, a problem occurs during running. After the getlogin () function is called, the returned result is NULL and the user name is not obtained (I log on to the linux system as a common user)
$./Getname
Getlogin: no such file or directory
What is going on?